Linda M. Wasiczko

Aperture averaging of optical scintillations in the atmosphere : experimental results

The aperture averaging factor is a measurement of the ratio of the atmospheric turbulence-induced irradiance scintillations incident on a receiver with diameter D to those incident on a point receiver. Characterizing the amount of aperture averaging in an optical receiver system will allow for tradeoffs to be made between the error performance of an optical receiver and the size and weight of the receiver. Aperture averaging theory has been extensively developed for optical wave propagation in weak turbulence conditions. A lack of experimental aperture averaging data over a variety of turbulence strengths, test range characteristics, and aperture diameters restricts progress in the development of useable propagation models. Experimental measurements of the aperture averaging factor collected over a test range at the University of Maryland are presented here. The impact of this data on the design of free space optical communication receivers will be discussed.

Atmospheric Turbulence Studies of a 16 km Maritime Path

The Naval Research Lab (NRL) is currently operating a lasercom test facility (LCTF) across the Chesaepeake Bay between NRLs Chesapeake Bay Detachment (NRL-CBD) and NRL-Tilghman Island. This lasercom test facility has successfully demonstrated 32 km retro-reflected links at data rates up to 2.5 Gbps. Along with lasercom link studies, atmospheric characterization of the NRL-CBD to Tilghman Island optical path has been investigated. These studies range from passive optical turbulence monitoring based on angle-of-arrival measurements of a spotlights apparent motion, to intensity and angle-of-arrival measurements of a retro-reflected laser beam. Currently the LCTF is being upgraded from a retro-reflected link to a direct one-way link from NRL-CBD to NRL-Tilghman Island. Initial measurements of atmospheric turbulence effects in this one-way configuration have recently been performed. Results of these past and current atmospheric turbulence studies are presented.

Multiple quantum well based modulating retroreflectors for inter- and intra-spacecraft communication

Free space optics (FSO) can provide high data rates with efficient use of power. However, small platforms may not be able to support the payload requirements of a conventional FSO terminal. An alternative FSO terminal uses a modulating retro-reflector (MRR). MRRs shift most of the power, weight, and pointing requirements to one end of the link. With a MRR configuration, it is possible to establish a two-way FSO link using a single laser transmitter. The MRR terminal of these systems can be small, lightweight, and low power. The MRR maintains the small beam divergence of a conventional optical communications link, but gains the loose pointing advantage of an RF link, reducing the pointing requirements. Communication needs in space present many asymmetric scenarios in which a MRR architecture could be beneficial. This paper describes some of the current capabilities and limitations of MRR systems, as well as applications to space links. An evaluation of the radiation tolerance of modulators is presented.

Latest results from the 32 km maritime lasercom link at the Naval Research Laboratory, Chesapeake Bay Lasercom Test Facility

The Naval Center for Space Technology at the Naval Research Laboratory reports the latest results from the long-range, maritime, free-space lasercom test facility located between Chesapeake Beach, MD and Tilghman Island, MD. The two sections of the facility are separated by 16.2 km of the Chesapeake Bay. Using a new OC-48 receiver developed by NRL’s Optical Science Division with a sensitivity of -33dBm for 10-9 bit error rate at 2.5 Gbps, we have closed a 32.4 km maritime lasercom link (round trip across the Chesapeake Bay) and performed bit error rate testing while transmitting 1.13 Terabytes of data. Bit error rate testing was also performed at lower data rates when atmospheric conditions were not favorable for high speed (2.5 Gbps), including testing at 150 Mbps through light fog and rain. In addition, we have set up a system for digitizing and transmitting full-color, uncompressed, video along with six audio channels and three RS-232 data channels over the maritime link. The digital link operated at 311 Mbps and could be maintained indefinitely, depending on atmospheric conditions. Several complete videos were transmitted in entirety or in part as well as live video from a handheld camcorder to test the system operation and robustness. The transmitter and receiver were co-located on the western shore of the bay at the NRL Chesapeake Bay Detachment. The data for both the bit error rate testing and the video was transmitted across the bay and returned from an array of retroreflectors located on a tower at Tilghman Island on the eastern shore. The lasercom links were closed with static pointing and with no active atmospheric aberration mitigation such as adaptive optics or fast steering mirrors on the receiver optics.

Lasercomm demonstration during US navy trident warrior 06 forcenet exercise

The purpose of the lasercomm demonstration during the Trident Warrior 06 (TW06) exercise was to explore the performance of a high data rate free space laser communication system in the operational Navy during a transit from San Diego, California, to Honolulu, Hawaii. This was a joint effort between the Naval Research Laboratory and NovaSol, Inc. and was the first demonstration of a high speed ship-to-ship lasercomm system between operational large deck Naval platforms. High data rate free-space laser communication systems have been previously demonstrated over terrestrial and air-to-ground paths.

Overview of NRL's maritime laser communication test facility

NRL has established a 20 mile round trip laser communication test facility across the Chesapeake Bay for investigating lasercomm performance in a maritime environment. Experiments at this facility have successfully demonstrated links at data rates up to 2.5 Gbps and at lower rates in light rain and fog. This facility is currently being upgraded to allow long term monitoring of a one-way 10 mile link across the Bay. Parameters monitored will include BER, turbulence conditions, atmospheric transmission, and meteorological conditions. A summary of past results, the design/status of the upgrade to the test facility, and recent results will be presented.

Characterization of the marine atmosphere for free-space optical communication

The Chesapeake Bay Detachment of the Naval Research Laboratory (NRL-CBD) provides an ideal environment for characterizing the effects of the marine atmosphere on free space optical communication links. The site has recently been converted to an operational 10 mile (16.2 km) one-way test range to collect information on propagation statistics in a variety of atmospheric conditions. The results presented here compare the contributions of thermal gradients across the bay to the variations in intensity scintillations across the bay.

Studies of atmospheric propagation in the maritime environment at NRL

The US Naval Research Laboratory has an ongoing research effort in the continuous observation of the maritime environment for free space optical communications. One of the goals of our research program is to characterize the behavior of the maritime environment for lasercomm systems, and use the data gathered from the Lasercomm Test Facility (LCTF) at NRL-Chesapeake Bay Detachment (CBD) to develop a method of predicting the global availability of maritime lasercomm. The LCTF has provided volumes of information about maritime laser propagation and atmospheric turbulence. Highlights of the work on the characterization of the maritime atmosphere are provided in this paper.

Large Diameter, High Speed InGaAs Receivers for Free-Space Lasercom

The U. S. Naval Research Laboratory (NRL) and OptoGration, Inc. have collaborated in the development and testing of large area, high speed InGaAs avalanche photodiode (APD) receivers for use in free-space lasercom systems. A 200 micron diameter InGaAs APD receiver has been tested in a free-space lasercom testbed and has demonstrated sensitivities of -42.4 dBm at 622 Mbps and -44.8 dBm at 155 Mbps. A 100 micron diameter receiver has been tested with a resulting sensitivity of -35.75 dBm at 2.4883 Gbps. These receivers are made possible due to OptoGrations capability to manufacture a large area, high speed InGaAs APD with an effective ionization ratio of < 0.2 and by matching the APD device with an appropriate transimpedance amplifier and limiting amplifier. Development and testing of the APD receivers will be described below.

Packet testing in free-space optical communication links over water

NRLs Chesapeake Bay lasercom test facility (LCTF) offers a variety of ranges for researching free-space optical laser communication (FSO lasercom) links in a maritime environment. This paper discusses link performance over the 16 km one-way range at the LCTF. There are several methods to determine the link quality in FSO lasercom. Bit-error-rate (BER) testing and packet testing are two possible methods. Since errors generally tend to occur in bursts in FSO channels, packet testing may offer a better indication of the quality of service (QoS) rather than BER testing. Link performance measured via packet testing is being investigated in a variety of atmospheric conditions. Results of these experiments will be presented.